The invention relates to benzoxazines, high char yield polybenzoxazines formed therefrom and method of forming said polybenzoxazines.
Benzoxazines are a class of phenolic resins that are generally formed by the reaction of a phenol, primary amine and formaldehyde or paraformaldehyde. The benzoxazine may be polymerized by simple heating to open the oxazine ring. This leads to the formation of a chemical bond between the phenolic groups. Cross-linked (i.e., thermoset) polybenzoxazines generally require multi-functional benzoxazine monomers (i.e., they contain more than two reactive sites). Generally, cross-linked polybenzoxazines are formed from benzoxazine monomers having more than one benzoxazine rings.
Benzoxazines exhibit several advantages for forming thermally stable polymers compared to resole phenolics and novolacs. For example, benzoxazines do not form any volatile reaction products upon polymerization, which is, for example, an advantage for forming composite parts (i.e., no out-gassing). In addition, they do not require the use of strong acid polymerization catalysts; such as used to cure resoles, which can cause corrosion of a mold.
One of the desirable traits of the cured benzoxazines is their potential to achieve high char yields, which are indicative of their thermal stability. However, many of the benzoxazine monomers that give cured products with high char yields often have limited processability because of their high viscosity below their reaction temperature. This makes them more suitable for processing methods, such as compression molding, rather than high speed processing methods, such as pultrusion. That is to say, they do not achieve desirable processing viscosities (i.e., sufficient to easily impregnate fiber matrices) without the aid of a solvent or substantial heating.
Unfortunately, the use of a solvent adds processing steps to remove it. The disadvantage of applying heat to the neat resin (benzoxazine) is that at the temperatures (e.g., about 160xc2x0 C.) needed to achieve a reasonable processing viscosity (e.g.,  less than 1000 centipoise) the benzoxazines autopolymerize resulting in a very short processing time. As a result, the viscosity builds faster than the ability to efficiently impregnate the fiber mat because of polymerization. Consequently, a method of obtaining both high char yields and good processability, without the use of solvents, is desirable for extending the utility of benzoxazines into applications, such as structural composites.
A recent attempt to increase the char yield of polybenzoxazines has been described by Ishida in WO 99/18092. Ishida describes incorporating a pendant reactive group, either to the phenolic or amine reactants, to create the benzoxazine monomer. The pendant reactive group is a group that is reactive with itself (e.g., acetylene group) or is a first reactive pendant group that is reactive with a second pendant reactive group, neither being reactive with itself. Unfortunately, even though these benzoxazines are reported to increase the char yield, they fail to address the processing problems described above.
Accordingly, it would be desirable to provide a polybenzoxazine that overcomes one or more of the problems of the prior art, such as one of those described above. It would be particularly desirable to provide an easily processed benzoxazine composition that still forms a high char yield polybenzoxazine.
A first aspect of the present invention is a polybenzoxazine comprised of the reaction product of a mixture comprised of (i) a benzoxazine compound, (ii) a furan compound, (iii) a benzoxazine-furan compound or (iv) combinations thereof, wherein the mixture has a furan ring to benzoxazine ring ratio of at least about 0.001 to at most about 10.
A second aspect of the invention is a method of forming a polybenzoxazine comprised of:
mixing (i) a benzoxazine compound, (ii) a furan compound, (iii) a benzoxazine-furan compound or (iv) combinations thereof, wherein the mixture has a furan ring to benzoxazine ring ratio of at least about 0.001 to at most about 10 and
heating the mixture for a sufficient time to form the polybenzoxazine.
A third aspect is a mixture useful for preparing a polybenzoxazine comprised of (i) a benzoxazine compound, (ii) a furan compound, (iii) a benzoxazine-furan compound or (iv) combinations thereof, wherein the mixture has a furan ring to benzoxazine ring ratio of at least about 0.001 to at most about 10.
In the broadest sense, the benzoxazine compound is a compound that contains at least one benzoxazine ring, but fails to have a furan ring. Similarly, the furan compound is a compound that contains at least one furan ring but fails to have a benzoxazine ring. Finally, the benzoxazine-furan compound contains at least one furan ring and at least one benzoxazine ring. Practically, each of the compounds, generally, has at most about 100 carbon atoms.
Surprisingly and unknown until the present invention, it has been discovered that a furan group, which does not react with itself, can be combined with a benzoxazine to give polymers with improved char yields after curing. Furthermore, it is unnecessary and may even be detrimental to add a compound containing a group that is reactive with the furan (i.e, maleimide) to the benzoxazine compound or benzoxazine-furan compound to obtain increased char yield. That is to say, the mixture preferably fails to have a benzoxazine compound that contains a maleimide group when the benzoxazine-furan compound is present in the mixture. It is even more preferable to have the mixture devoid of any maleimide.
In addition, the invention allows, for example, the use of certain benzoxazine-furan compounds that have only one benzoxazine ring and one furan group to unexpectedly form a thermoset polymer by itself. This type of compound also has been found to reduce the viscosity of a benzoxazine compound or benzoxazine-furan compound containing more than one benzoxazine ring without being detrimental to the final properties, such as char of the resultant polybenzoxazine.
These new polybenzoxazines may be used in the same applications as polymers, such as polyimide, phenolics, epoxies and other thermally stable polymers. In particular, they may be used in applications that take advantage of properties, such as fire resistance and low water absorption. Examples of applications include brake pads or composite applications, such as interiors and structural components of an aircraft, piping, ladders, ship masts, gratings, table trays, ducts, beams, ship housings, and printed wiring boards.
The polybenzoxazine is comprised of the reaction product of a mixture comprised of (i) a benzoxazine compound, (ii) a furan compound, (iii) a benzoxazine-furan compound or (iv) combinations thereof, wherein the mixture has a furan ring to benzoxazine ring ratio of at least about 0.001 to at most about 10.
Because of the furan ring to benzoxazine ring ratio limitation, it is understood that the mixture must contain at least some amount of either the furan compound or benzoxazine-furan compound. That is to say, one of these has to be present in the mixture to provide the furan ring required by the furan/benzoxazine ring ratio. The mixture preferably contains the benzoxazine compound and the furan compound. In addition, it is preferred to have a benzoxazine compound or benzoxazine-furan compound that has more than one benzoxazine ring in the mixture. More preferably, the mixture contains a benzoxazine-furan compound that has at least two benzoxazine rings and at least one furan ring. Even more preferably, the mixture contains a benzoxazine-furan compound having at least two benzoxazine rings, in combination with a benzoxazine-furan compound that has one benzoxazine ring and one or more furan rings.
To reiterate, the ratio of furan to benzoxazine rings in the mixture is at least about 0.001 to at most about 10. Preferably, the ratio is at least about 0.1, more preferably at least about 0.2, even more preferably at least about 0.4 and most preferably at least about 0.5 to preferably at most about 8, more preferably at most about 7, even more preferably at most about 5 and most preferably at most about 4.
The benzoxazine compound may be any suitable benzoxazine, such as those known in the art. The benzoxazine may be, for example, those described by U.S. Pat. No. 5,543,516; WO 99/18092 and Japanese Patent Application HEI 9-59333.
Preferred benzoxazine compounds include those of the formula: 
where R4 is a divalent radical that is (a) an aliphatic, aromatic or combination thereof and composed of 1-35 carbon atoms, together with up to five atoms selected from the group consisting of oxygen, nitrogen, sulfur, phosphorous, halogen and combinations thereof; (b) a single bond or (c) S, S2, SO, SO2, O or CO and R5 is one or more H, substituted or unsubstituted alkyl of 1 to 24 carbon atoms, or aromatic or substituted aromatic or aromatic substituted alkyl of 6 to 24 carbon atoms. Preferably, R4 is a single carbon that has bonded to it one or more groups, such as H, CH3, C2H5, C3H7, n-C3H7, i-C3H7, cyclohexyl, bicyclohexyl(2.2.1)heptyl, phenyl, CF2, CF3, CCl3, CF2Cl, CN, (CH2)2COOH3 or PO(OCH3)2. More preferably, R4 is one carbon directly bonded to two CH3 groups or two hydrogens. Preferably, R5 is an unsubstituted alkyl of 1 to 24 carbon atoms or unsubstituted aryl of 6 to 24 carbon atoms. More preferably, R5 is a 6 to 12 unsubstituted aryl. Most preferably, R5 is a phenyl.
R5 may also contain at least one reactive group described previously. The reactive group may be, other than a furan group, a group such as an alkene (i.e., Cxe2x95x90C), alkyne (i.e., Cxe2x89xa1C), nitrile (i.e, Cxe2x89xa1N), oxazole, oxazoline, phenol, epoxy, anhydride or imide. Examples of preferred R5 structures are: 
where R1 is one or more H, halogen, substituted or unsubstituted alkyl of 1 to 6 carbon atoms, or aromatic or substituted aromatic (e.g., a benzoxazine, but not a furan) or aromatic substituted alkyl of 6 to 24 carbon atoms and R2 is an alkyl of 1 to 5 carbon atoms, a halogen, a phenyl or combinations thereof. R1 may be a cyclic, linear or branched alkyl. R1 may be substituted with a group containing 1-3 atoms of O, N, P, S, halogen or combinations thereof. Preferably, R1 is H, OH, a substituted aromatic or aromatic substituted alkyl of 6 to 18 carbon atoms, more preferably of 6 to 12 carbon atoms. More preferably, R1 is H, OH, unsubstituted or substituted alkyl having 1 to 6 carbons. Most preferably, R1 is H or OH.
Generally, the furan compound is given by the formula: 
wherein X is composed of a total of 1-50 carbon atoms and up to 10 atoms selected from the group consisting of oxygen, nitrogen, sulfur, phosphorous, halogen and combinations thereof. Preferably, X is composed of at most about 30, more preferably at most about 25, even more preferably at most about 20, and most preferably at most about 18 carbon atoms. Preferably, X contains at most about 9, more preferably at most about 8, even more preferably at most about 7 and most preferably at most about 6 atoms selected from the group consisting of oxygen, nitrogen, sulfur, phosphorous, halogen and combinations thereof.
Preferably X contains a reactive group. The reactive group is a group that reacts with either another reactive group (e.g., an alkene reacting with another alkene) other than the furan or with the benzoxazine of the benzoxazine compound. Thus, a furan ring is believed to be a reactive group and is treated as such herein because of the belief that it reacts with the benzoxazine ring. The other reactive group may be another furan group or may be a group, such as an alkene (i.e., Cxe2x95x90C), alkyne (i.e., Cxe2x89xa1C), nitrile (i.e, Cxe2x89xa1N), oxazole, oxazoline, phenol, epoxy, anhydride and imide. Reactive group examples, besides the furan, include: 
where R1 and R2 are the same as described above.
Preferred furan compounds include (1) compounds of a reaction between an aldehyde or ketone-containing furan group with a primary amine (i.e., Schiff base), such as those described by Hui, et al., Eur. Polym. J., Vol., 28, No. 12, 1461-1469 (1992); (2) hydrogenated Schiff bases described by (1); (3) compounds of a reaction between a polyol (e.g., diol and triol) and an aldehyde containing a furan (e.g., furfural) to form, for example, an acetal, such as those described by Gousse, et al., Polymer International, Vol. 48, 723-741 (1999); (4) a polymer containing a furan described by Laita, et al., Eur. Polym. J., Vol. 8, 1203-1211 (1997) and (5) Gousse, et al., Macromolecule, Vol. 31, 314-321 (1998).
To reiterate, the benzoxazine-furan compound has at least one benzoxazine ring and at least one furan, such as those shown in the formula: 
wherein X is as described previously, Ar is a benzene ring or 2 to 4 fused benzene rings, j is an integer from 4-10 and Y is independently a monovalent radical that is H; OH; halogen (e.g., fluorine, bromine, chlorine) or an alkyl, aryl, alkenyl, alkynyl or combination thereof composed of 1-35 carbon atoms containing up to 5 atoms selected from the group consisting of oxygen, nitrogen, sulfur, phosphorous, halogen and combinations thereof.
Although Ar may be a benzene ring, or 2 to 4 fused benzene rings, Ar is preferably the benzene ring or 2 fused benzene rings (i.e., naphthalene-type structure). Most preferably, Ar is a benzene ring.
When Y is an alkyl, aryl, alkenyl and alkynyl or combination thereof, Y preferably contains at most about 24, more preferably at most about 18 and most preferably at most about 12 carbon atoms. It is also understood that this monovalent radical may have combinations of alkyl, aryl, alkenyl and alkynyl (i.e., saturated, unsaturated and aromatic double bonds). Y may also be cyclic, linear or branched. When Y has an aryl constituent, the aryl may be heteroaromatic, such as aromatic compounds containing, for example, O, S, N or combinations thereof. That is to say, Y may contain a benzoxazine and furan group.
Preferred benzoxazine-furan compounds include those with the formulas: 
where R3 is a single bond or substituted or unsubstituted alkyl group having from 1-6 carbon atoms and R7 is one or more H, halogen, substituted or unsubstituted alkyl of 1 to 24 carbon atoms, or aromatic or substituted aromatic (e.g., contains a benzoxazine or furan group) or aromatic substituted alkyl of 6 to 24 carbon atoms. R7 may be a cyclic, linear or branched alkyl. R7 may be a group containing 1-12 atoms of O, N, P, S, halogen or combinations thereof. Preferably, R7 is H, a substituted aromatic or aromatic substituted alkyl of 6 to 24 carbon atoms, more preferably of 6 to 12 carbon atoms. More preferably, R7 is H, an unsubstituted or substituted alkyl having 1 to 6 carbons. Most preferably, R7 is H or an unsubstituted alkyl having 1 to 6 carbons. Preferably, R3 is a C1 to C6 unsubstituted alkyl. More preferably, R3 is CH2 or C2H4. Most preferably, R3 is CH2.
More preferred benzoxazine-furan compounds include compounds having the following formulas: 
where R3 and R4 are as described previously and R7 is as described previously, with the proviso that it is devoid of a benzoxazine and furan group.
Preferred benzoxazine-furan compounds containing one benzoxazine and one furan (i.e., R7 lacks a benzoxazine and furan group and the benzoxazine-furan compound is not of an unsubstituted phenol) have surprisingly been found to not only form a thermoset cross-linked polymer, but also reduce the viscosity, particularly at temperatures below the temperature of polymerization, of a benzoxazine or benzoxazine-furan compound having at least two benzoxazine rings without being detrimental to properties, such as char yield. Even more surprising is that the addition of these to a benzoxazine or benzoxazine-furan compound containing more than one benzoxazine ring improves properties, such as char yield (i.e., increase char yield) of the resultant polybenzoxazine. These improvements are more readily observed when the addition is to the benzoxazine compound having more than one benzoxazine ring.
Generally, the benzoxazine or benzoxazine-furan compound is made by reacting a phenol, primary amine and formaldehyde or paraformaldehyde. Examples of phenols suitable in making the benzoxazine include those having the formula: 
wherein Ar is a benzene ring or 2 to 4 fused benzene rings and 1 is an integer from 6-12 and Y is independently a monovalent radical that is H; OH; halogen (e.g., fluorine, bromine, chlorine) or an alkyl, aryl, alkenyl, alkynyl, or combination thereof, composed of 1-35 carbon atoms containing up to 5 atoms selected from the group consisting of oxygen, nitrogen, sulfur, phosphorous, halogen and combinations thereof, with the proviso that at least one Y is OH that has an H ortho thereto.
Although Ar may be a benzene ring or 2 to 4 fused benzene rings, Ar is preferably the benzene ring or 2 fused benzene rings (i.e., naphthalene type structure). Most preferably, the Ar is a benzene ring.
When Y is an alkyl, aryl, alkenyl and alkynyl or combination thereof, the radical preferably contains at most about 24, more preferably at most about 18 and most preferably at most about 12 carbon atoms. It is also understood that this monovalent radical may have combinations of alkyl, aryl, alkenyl and alkynyl (i.e., saturated, unsaturated and aromatic double bonds). Y may also be cyclic, linear or branched. When Y has an aryl constituent, the aryl may be heteroaromatic, such as aromatic compounds containing, for example, O, S, N or combinations thereof.
Examples of monovalent Y radicals include reactive groups described previously. Preferably, Y is H, OH or an alkyl of at most about 6 carbons. More preferably, Y, other than the one required OH, is H.
Preferred phenolic compounds have the formulas: 
where R4 and R5 are the same as previously defined. Examples of phenols include phenol; cresol; 2-bromo-4-methylphenol; 2 allylphenol; 1,4-aminophenol; phenolphthalein; 2,2xe2x80x2-biphenol; 4,4xe2x80x2-methylene-diphenol; 4,4xe2x80x2-dihydroxybenzophenone; bisphenol A; 1,8-dihydroxyanthraquinone; 1,6-dihydroxnaphthalene; 2,2xe2x80x2-dihydroxyazobenzene; resorcinol; fluorene bisphenol; 1,3,5-trihydroxy benzene; novolac resin and polyvinyl phenol.
The primary amine may be any suitable amine to react with the phenolic compound, such as those known in the art. It is preferred that a reactive group, such as those described above, is bonded to the nitrogen. More preferably, the primary amine has the formula: 
where R6 is a single bond or a divalent alkyl radical having from 1 to 6 carbon atoms. Preferably, the primary amine is furfurylamine.
Other examples of amines include monofunctional amines (i.e., one amine) and include those with up to 40 carbon atoms, such as ammonia; methylamine; 5-methylfurfurylamine; ethylamine; propylamine; butylamine; isopropylamine; octadecylamine; cyclohexylamine; alkylamine; 1 aminoanthracene; 4-aminobenzaldehyde; 4-aminobenzophenone; aminobiphenyl; 2-amino-5-bromopyridine; 3-amino-xcex5-caprolactam; 2-amino-2,6-dimethylpiperidine; 3-amino-9-ethylcarbazole; 4-(2-aminoethyl)morpholine; 2-aminofluorenone; 2-aminofluorene; 1-aminohomopiperidine; 9-aminophenanthrene; 1-aminopyrene; 4-bromoaniline and aniline.
Examples of multifunctional amines (i.e., more than one amine) include those having up to 40 carbon atoms, such as 2-aminobenzylamine; 1,3-diaminopropane; 1,4-diaminobutane; 1,10-diaminodecane, 2,7-diaminofluorene; 1,4-diaminocyclohexane; 9,10-diaminophenanthrene; 1,4-diaminopiperizine; 4,4xe2x80x2-methylenedianiline; 4,4xe2x80x2-diaminobenzophenone; diaminodiphenylsulfone; diaminodiphenylsulfide; 4,4xe2x80x2-oxydianiline; melamine; fluorene-tetraamine and tetraaminediphenylether.
The benzoxazine or benzoxazine-furan compound may be made by any suitable method, such as those known in the art. Exemplary methods include those described by U.S. Pat. Nos. 5,543,516 and 5,152,939; WO 99/18092; Japanese Patent Application Nos. Hei 9-59333 and Hei 8-74896 and Swiss Patent A5 11 606169. Generally, the benzoxazine is made by reacting a phenolic compound, such as those described above with a primary amine and formaldehyde, paraformaldehyde or combination thereof in an equivalent ratio of phenol, amine and aldehyde of about 1:1:2. Variations from this equivalent ratio may be used such as those described by Swiss Patent A5 11 606169.
In making the benzoxazine, a phenol, primary amine and formaldehyde or paraformaldehyde are combined with or without a solvent. This mixture is then heated to a temperature sufficient to form the benzoxazine. Generally, the temperature is from about 70xc2x0 C. to about 100xc2x0 C. Preferably, the temperature is at least about 80xc2x0 C. After cooling, if a solvent is used, it may be removed by any suitable method, such as those known in the art. Examples of suitable solvents are dioxane, toluene, n-butyl acetate and methylisobutyl ketone.
The polybenzoxazine is generally made by heating the mixture compound to a temperature for a time sufficient to form the polybenzoxazine. The temperature is desirably at least about 70xc2x0 C. to at most about 300xc2x0 C. Preferably, the temperature is at least about 100xc2x0 C. and more preferably at least about 150xc2x0 C. to preferably at most about 250xc2x0 C. and more preferably at most about 225xc2x0 C. Generally, the time at temperature to form the polybenzoxazine is from about 1 minute to about 20 hours.
Polymerization of the benzoxazines is usually done without the use of a solvent, however, a solvent may be used, for example, to facilitate application of the monomer to a substrate. Examples of solvents that may be used include dioxane, tetrahydrofuran, acetone, dimethylformamide, toluene, n-butyl acetate and methylisobutyl ketone.
Even though a catalyst is not necessary, one may be used. Typical catalysts are described in J. Appl. Polym. Sci., 1995, 58, 1751-1760; J. Polym. Sci, Part A: Polym. Chem., 1999, 37, 1913-1921; Polymer, 1999, 40, 4563-4570; Poly. Mat. Sci. Eng., 1999, 81, 114-115 and GB 1 437 814 and include compounds, such as acids, phenols, novolacs, Lewis acids and bases. When a catalyst or initiator is used, it is employed in small quantities, such as 0.01 percent to about 5 percent by weight of the mixture.
When forming the polybenzoxazine, another additive may also be added for reasons, such as color, fire retardancy, mechanical properties, processing properties and electrical properties. An example of an additive is an epoxy resin, such as those known in the art.